1. Field of the Invention
The present invention relates to a servo controller for controlling an AC servo motor and an AC spindle motor used as a driving source for a machine tool, an industrial machine, a robot and the like which are controlled by a numerical control (NC) device.
2. Description of the Related Art
A machine tool sometimes uses a synchronous motor for a feed axis for positioning and an induction motor for a spindle in combination. Further, in a vertical-type lathe, when a particular table-feed axis is to be driven in a machining mode putting emphasis on power and also in a positioning mode putting emphasis on efficiency, a synchronous motor and an induction motor are sometimes used by being changed over for each mode.
Generally, in driving the synchronous motor, position of the magnetic pole is determined by using an optical absolute position sensor. In driving the induction motor, the absolute position sensor required for the synchronous motor is unnecessary and an incremental position senor for detecting a relative position can sufficiently carry out the control.
As described above, in order to servo-control the motors which require to be driven in different control modes, such as the synchronous motor and the induction motor, it is necessary to use a servo controller dedicated to each motor and a sensor associated with the motor.
For this reason, when providing servo controllers for maintenance and replacement for a machine having both of the synchronous motor and the induction motor, servo controllers each dedicated to the synchronous motor or the induction motor have to be made ready, which raises problems in terms of maintenance management and cost.
When the synchronous motor and the induction motor are used by being changed over, a servo controller has to be provided for each motor, which increases the number of axes controlled by an NC device and causes problems of an increase in installation area and cost.
FIG. 11 is a block diagram showing an arrangement in which a plurality of motors to be driven in a different control mode are controlled by individual servo controllers. In FIG. 11, a first motor (synchronous motor) Ma is controlled by a servo controller 1a for the first motor, and a second motor (induction motor) Mb is controlled by a servo controller 1b for the second motor. The servo controller 1a for the first motor and the servo controller 1b for the second motor each carry out position control and/or velocity control by using signals fed back from the first motor Ma and the second motor Mb, respectively. In this servo control, the induction motor does not require absolute position information in the feedback signals, so that a relative position sensor suffices for the installed sensor. In contrast, the synchronous motor requires absolute position information in the feedback signals, so that an absolute position sensor is needed.
As the absolute position sensor is constituted by optical mechanisms using a glass plate or the like, it has a problem of being easily affected by the operation environment such as vibration and dust. In contrast, as the relative position sensor which generates incremental signals may be a gear-type sensor or the like, it has an advantage of high reliability against vibrations at the time of high-velocity rotation. Also, a through hole is sometimes formed at the center of the motor for cooling etc. In such a case, a through hole can be formed easily at the center of the gear-type sensor.
The servo controller 1a for the first motor and the servo controller 1b for the second motor are supplied with a position command and/or a velocity command from a CNC 2. Also, a PC (programmable controller) 3 operates by a control signal from the CNC 2 for controlling electric circuits 4 for peripheral equipments.
As described above, when servo-controlling the motors having different control modes, such as the synchronous motor and the induction motor by the conventional servo controller, each motor requires a dedicated servo controller to raise problems in terms of maintenance, increase in size and installation area of the devices, cost, load on the CNC, etc.